This invention relates to a driving device for an ultrasonic actuator, particularly to a driving device capable of a position servo control in an ultrasonic actuator driven in a specified resonant state, and also relates to a position controller provided with a driving device, and a camera provided with a position controller.
In recent years, cameras such as still cameras, digital cameras and camera-integrated VTRs have been required to be smaller and more power-saving for portability and mobility. Thus, driving devices for driving focusing lenses, zoom lenses and camera shake correction lenses have been required to be smaller and more power-saving. Ultrasonic actuators using electromechanical conversion elements have been studied and developed for the use in driving devices to meet this request.
One of the ultrasonic actuators is a piezoelectric actuator constructed such that a movable member having a drivable object to be driven such as a lens mounted thereon is so engaged with a bar-shaped driving member as to have a specified frictional force, and a piezoelectric element is secured to one end of the movable member. In the case of using the thus constructed piezoelectric actuator to drive a focusing lens, a zoom lens, a camera shake correction lens or the like, operative states such as the position, speed and acceleration of the movable member need to be controlled for precise focusing, magnification adjustment and camera shake correction. Particularly, in the case of driving the camera shake correction lens, camera shake needs to be continuously corrected during a specified period, e.g., a period lasting until an exposure is completed after a release button is fully pressed. Thus, it is particularly important to servo-control the position of the movable member so as to pursue a continuously changing target position with a minimum deviation.
A driving device for controlling an ultrasonic motor as one of the ultrasonic actuators is disclosed in Japanese Unexamined Patent Publication No. 2001-078472. However, the driving device disclosed in this publication controls the frequency while detecting a phase difference so that a disk-shaped ultrasonic motor can be constantly driven at a mechanical resonance frequency and is not intended to control the position of a vibrating body which corresponds to the movable member.
Further, Japanese Unexamined Patent Publication No. 8-201402 discloses a position control system for a scan-type probe microscope which system controls the position of a probe using a piezoelectric actuator to drive the probe. However, the piezoelectric actuator disclosed in this publication is, according to Japanese Unexamined Patent Publication No. 6-229753, constructed such that a single common electrode is provided on the inner circumferential surface of a tubular piezoelectric element while four drive electrodes are circumferentially provided on the outer circumferential surface thereof, voltages applied to the four drive electrodes are suitably controlled to three-dimensionally displace a free end of the piezoelectric element by bending, elongating and shrinking. A stage is secured to the free end of this piezoelectric element and a sample is mounted on this stage, whereby the sample is scanned by a probe by the displacement of the free end of the piezoelectric element. Accordingly, the position control system for the piezoelectric actuator disclosed in the above publication is for controllably moving the probe to a desired position, but not for servo-controlling the position of the probe. Nor is this system for controlling the position using a resonant state.
A driving circuit for suitably controlling the speed, acceleration and the like upon the position servo in such a state where the ultrasonic actuator is driven in the specified resonant state has not been known as described above. Thus, there has been a demand for a driving device capable of a position servo control in the case that the position of a drivable object needs to be controllably adjusted to a continuously changing control target position.